Studies at the National Institutes of Health (NIH) of transfusion- associated hepatitis and at the Centers for Disease Control and Prevention (CDC) of community-acquired hepatitis indicated that 10 to 20 percent of observed hepatitis cases could not be accounted for by the known hepatitis viruses: A, B, C, D, and E. To investigate the existence of an additional agent or agents of human hepatitis, a collaborative research and development agreement was established between the Department of Transfusion Medicine, NIH; Genelabs Technologies, Inc.; and CDC. Suspect clinical materials were sent to Genelabs and were amplified following reverse transcription by sequence independent single primer amplification and then cloned into Escherichia coli, using a gt11 expression vector. A clone was identified that was novel to the gene bank and exogenous by hybridization assays. The agent could be recovered from the cloning source and was shown to be a flavivirus distinct from other members of that family. It had approximately 25 percent homology with hepatitis C virus (HCV). A polymerase chain reaction assay was developed, and clinical materials were screened. The agent, tentatively designated HCV, is associated with approximately 20 percent of transfusion hepatitis and community-associated hepatitis that are unrelated to previously recognized agents, but its causal role in the development of hepatitis has not been established. HGV is found in 1 to 2 percent of blood donors and has proved to be transfusion transmitted by linked donor-recipient samples. In summary, HCV is a new member of the flaviviridae family that is prevalent in the general population and is transfusion transmissible. Further studies are required to establish its disease burden and, particularly, to define whether it causes hepatitis or is merely associated with hepatitis because of shared transmission patterns with another agent(s) that is currently undetectable.